feat(series): optimize locale-focused image responses

[wax911]

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Description

Optimizes v1/series image payloads by keeping cached canonical SeriesDocument data intact while returning locale-focused images in the HTTP response.

This change adds deterministic image selection that prefers Japanese artwork first, then the device language, and finally universal (null) images for locale-specific requests. It intentionally stops before unrelated locales, so requests like de-DE no longer surface assets such as ru when no German image exists. Requests without a locale still keep the broader fallback behavior so the endpoint can return a useful secondary image when available.

The controller now reads client locale from request context and passes it into SeriesService, where response-only filtering happens after repository retrieval. The series resolver also distinguishes true not-found cases from ARM upstream lookup failures with typed errors so ARM failures continue to surface as server errors rather than 404 responses.

Tests cover the image-selection helper, service integration, controller locale pass-through, ARM lookup error handling, and broader series module verification. Documentation was added under docs/superpowers/specs/2026-05-25-series-endpoint-image-optimization-design.md and docs/superpowers/plans/2026-05-25-series-endpoint-image-optimization.md.

Verification run for this branch:

• deno task test -- --filter "series"
• deno task check
• deno task lint
• manual local curl validation for no locale, en-US, ja-JP, and de-DE requests

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